Two channel Kondo effect and non Fermi liquid state in diluted alloy of cerium hexaboride

CexLa1-xB6是一个近藤系统，其中4f电子的基晶电场状态(伽马8)同时带有磁偶极自由度和电四极自由度。众所周知，当Ce浓度很低时，CexLa1-xB6的基态是局域费米液体。通过测量CexLa1-xB6在很低温度下的电阻率，发现随着Ce浓度的增加，CexLa1-xB6的基态由局域费米液态转变为非费米液态。这一行为与CexLa1-xCu6不同，在CexLa1-xCu6中，从局部费米液体逐渐转变为重费米子。在CexLa1-xB6的情况下，这种非费米液体在强磁场中是不稳定的。CexLa1-xB6的电阻率在各个Ce浓度下都表现为高场下的费米液体行为。我们已经通过对德哈斯-范阿尔芬(DHvA)效应的观察证实了这一点。在每个Ce浓度下都观察到dHvA振荡。得到的有效质量与Ce浓度有很好的比例关系，并且随磁场的减小而发散。当Ce浓度x大于0.6时，零场下的CexLa1-xB6经历了从非费米液体到长程有序态的量子相变。在长程有序态下，其电阻率表现为费米液体行为。在本研究中，我们发现在CexLa1-xB6系统中，高磁场和长程有序稳定了费米液态。这表明CexLa1-xB6中非费米液体的起源是4f电子局域力矩的涨落。

CexLa1-xB6 is a Kondo system where the ground crystal electric field state of 4f electrons (a Gamma 8) carries both magnetic dipolar and electric quadrupolar degrees of freedom. It is well known that the ground state of the CexLa1-xB6 is a local Fermi liquid when the Ce concentration is very low. We have investigated the electronic phase diagram of this system with changing the Ce concentration and magnetic fields.Through the measurements of electrical resistivity at very low temperature, we have found that the ground state of CexLa1-xB6 changes from the local Fermi liquid state to a non Fermi liquid state with increasing the Ce concentration. This behavior is quite different from that of CexLa1-xCu6 where the gradual change from a local Fermi liquid to a heavy fermion occurs. In the case of CexLa1-xB6, this non Fermi liquid is instable in high magnetic fields. The electrical resistivity of CexLa1-xB6 shows Fermi liquid behavior under high fields at every Ce concentration. We have confirmed this point by the observation of the de Haas-van Alphen (dHvA) effect. The dHvA oscillation is observed in every Ce concentration. The effective mass obtained is well scaled by the Ce concentration and the mass diverges with decreasing the magnetic fields. This probably indicates that the Fermi liquid state becomes instable with decreasing the magnetic fields.When the Ce concentration x becomes higher than 0.6, CexLa1-xB6 at zero field undergoes a quantum phase transition from the non Fermi liquid to a long range ordered state. In the long range ordered state the electrical resistivity shows Fermi liquid behavior. In the present study we have found that the high magnetic fields and the long range ordering stabilize the Fermi liquid state in the CexLa1-xB6 system. This suggests that the origin of the non Fermi liquid in CexLa1-xB6 is the fluctuation of the local moments of 4f electrons.